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In quantum physics, a particle can be in a superposition of two states until it is measured. In other words, the aforementioned particle doesn't have a definite state until it is "looked at" (measured). Since certain properties (i.e., an electron's position) of a particle aren't well defined until they are measured, does this mean that quantum objects don't possess these properties, unless they are looked at? Do quantum objects even exist without an observer? Since macroscopic things are made of quantum particles, does the moon stop existing when no one is looking at it, since its particles don't have a definite position, momentum, etc.? Do the answers to these questions depend on the interpretation of quantum physics being used?

I guess my main question is this: Is there an external, objective reality that exists even when it is not being measured?

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  • $\begingroup$ First of all, these questions aren't really under the scope of physics. Science tells you what Nature does, and only offers metaphors of why and how. Second, ask yourself why you find it so weird that particles don't have definite momentum and position. So what? Momentum and position are made up concepts which we use to accurately describe results of experiments. So is the wave function. Quantum physics is really no weirder than any other physics theory, it's just slightly less intuitive and because of that people scrutinize it in ways they never bother to scrutinize all the other theories. $\endgroup$ – DanielSank Oct 14 '14 at 6:05
  • $\begingroup$ @DanielSank Ok, fair enough. So my main question--Is there an external, observer independent reality--isn't under the scope of physics? Is that right? $\endgroup$ – Jmfig314 Oct 14 '14 at 6:11
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    $\begingroup$ I would say @DanielSank is adopting an 'empiricist' interpretation of science: basically if you can't answer your question through experiment then it's not science. It's a reasonable enough position, but it's far from universally accepted. $\endgroup$ – Mark A Oct 14 '14 at 6:29
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    $\begingroup$ @MarkA: I think it's universally accepted that "science" deals with things which are falsifiable. As far as I can tell, that is congruent with "can be checked through experiment." That said, trying to understand what is and isn't "on-topic" on this site boggles the mind. All that aside, I think my point about subjecting "simpler" theories to the same philosophical inspection as we do to quantum mechanics is a good and relevant one. $\endgroup$ – DanielSank Oct 14 '14 at 6:55
  • $\begingroup$ @Jmfig314: I would say the question you're asking isn't even well formed. What does "observer independent reality" mean to you? I challenge you to even come up with a reasonably good scientific definition of "reality". This is quickly going to become philosophy, by the way, so we should stop doing this on a physics Q&A site. If you post on reddit/askscience I'll probably see it. $\endgroup$ – DanielSank Oct 14 '14 at 7:10
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A superposition is a perfectly valid state in quantum mechanics. It simply doesn't have a correspondence in classical physics, which is the result of quantum mechanics (and not the other way around!).

"In other words, the aforementioned particle doesn't have a definite state until it is "looked at" (measured)."

Yes, the particle (or better "the quantum system") does have a definite state: it's the superposition of two or more eigenstates.

"Since certain properties (i.e., an electron's position) of a particle aren't well defined until they are measured"

They are perfectly well defined at all times. It's the act of a STRONG measurement which reduces the state to a pure state. There are weak measurements which do not. After the measurement the state is equally well defined, it's just not the same state as the system had before the measurement.

All of this is perfectly "objective", it's just very counterintuitive to those who can't accept that the world is not classical. The world was NEVER classical, we merely have been looking at the world using the wrong description until we discovered the right one approx. 80 and some years ago.

"Do quantum objects even exist without an observer?"

No more and no less than a falling tree makes a sound when nobody listens. That's an incorrectly posed question even in classical physics.

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  • $\begingroup$ Alright, I think I get it now. Thanks for clearing up my confusion. $\endgroup$ – Jmfig314 Oct 16 '14 at 16:07
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In quantum physics, a particle can be in a superposition of two states until it is measured. In other words, the aforementioned particle doesn't have a definite state until it is "looked at" (measured).

A superposition state is a state. So a particle in such a state does have a state before being measured.

Since certain properties (i.e., an electron's position) of a particle aren't well defined until they are measured, does this mean that quantum objects don't possess these properties, unless they are looked at?

No. It means that the particle doesn't have a single position. Rather there are multiple instances of it at different positions. Those instances can interact with one another in interference experiments, which is why you can't understand what it is doing by considering it as having only a single position. See

http://arxiv.org/abs/quant-ph/0104033

Do quantum objects even exist without an observer? Since macroscopic things are made of quantum particles, does the moon stop existing when no one is looking at it, since its particles don't have a definite position, momentum, etc.?

For a large non-isolated system like the moon, interactions with other systems prevent it from undergoing interference on a macroscopic scale. It does exist in multiple versions but you can't see the other versions, see

http://arxiv.org/abs/quant-ph/9612037

Do the answers to these questions depend on the interpretation of quantum physics being used?

Different "interpretations" of quantum mechanics make different claims about what is happening in reality. So they ought to be treated as different physical theories, not interpretations. Quantum mechanics without the collapse postulate implies the Everett interpretation:

http://www.daviddeutsch.org.uk/many-minds-interpretations-of-quantum-mechanics/

Other interpretations may have different implications about how the world works but they are usually ambiguous about what exists and some are apparently inconsistent or otherwise unviable.

I guess my main question is this: Is there an external, objective reality that exists even when it is not being measured?

Yes.

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The question you ask is a tough one and everyone has his own opinion about the answer (look at the comments of your question). In particular, one has to adopt a more or less clear philosophical position about what science ought to tell us in order to reply.

That being said, the decoherence programme tries to address some of the questions you wonder about. In particular, classicality is supposed to emerge via a "measurement by the environment" that projects a quantum system that ought to behave classically (like the Schrodinger's cat) onto a (quasi-orthonormal) pointer basis that so happens to correspond to the classical observables we are used to (e.g. a phase space projector or the dead or alive states for the cat).

In this process of einselection, it is often thought that there is not really an objective way of determining the actual pointer states.

Now, quite recently Zurek et al. have proposed a solution to the idea of objective pointer states (that has been quite acknowledged in the literature) which they have called quantum Darwinism.

My contribution would not be complete if I didn't give you a reference claiming that there is problem with the decoherence programme and in particular with the idea of quantum Darwinism. More about it here.

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